1. Field of the Invention
The present invention relates to battery chargers and has particular though not exclusive application to chargers for use with mobile phones.
2. Description of the Related Art
Reference is made to the following publications which provide a background to the present invention:
U.S. Pat. No. 4,392,101 to Saar measures the rates of change of voltage during charging and determines the inflection points on the characteristic voltage versus time curve by means of a change in the sign of the second derivative of voltage.
U.S. Pat. No. 5,818,204 to Banyas et al is concerned with the charging of batteries having a temperature significantly different from the temperature of the charging environment. A timer is used to compensate for the loss of a temperature sensor within the battery pack by delaying the start of charging until the difference in temperature between battery and environment has reduced sufficiently.
U.S. Pat. No. 5,519,303 to Goedkin et al describes a battery charger control circuit which disables charging through detection of a change from negative to positive of the second differential of temperature coupled with reaching a predetermined threshold of the first derivative of temperature.
The rate at which a battery is charged is denoted by the factor C where C represents the charge current required to charge a battery in one hour. On this basis 1 C might be considered as a fast charging rate and 1/3 C as a medium charging rate. A very low charging rate, so called trickle charge would be perhaps 1/40 C.
The rates of change of voltage and temperature of a battery whilst charging will depend upon the charging rate. Both voltage and temperature will increase more quickly at a higher charging rate, say 1 C, than at a lower charging rate, say 1/3 C.
A graph of battery temperature versus charging time for most battery types will show a characteristic curve and reference is made to FIG. 1 which is a characteristic curve for a nickel metal hydride (NiMH) battery. A similar characteristic curve will apply to a nickel cadmium (NiCd) battery.
Measurements of the battery temperature taken during the charging process should provide an indication of the point on the characteristic curve reached by the battery at any time. When the point on the curve representing full charge of the battery is reached charging should cease.
The temperature can be measured continuously or can be sampled. Where the current state of charge can be determined with confidence, sampling can sometimes be discontinued or reduced over much of the charging cycle. Because of the variations in the state of different batteries, however, accurate and precise determination of the current state of charge relating to points on the characteristic temperature curve is difficult.
Variations in the state of different batteries offered for charging are introduced by a number of factors including the existing state of charge of the battery, a battery temperature much different to that of the ambient temperature of the charging circuit environment, the age of the battery, the number of re-charging cycles previously applied to the battery and previous use or misuse of the battery.
The state of charge of a battery connected to a charging circuit may range from a completely discharged battery to a completely charged battery. Determination of the state of charge of a battery solely by means of a measurement of the voltage across the battery terminals can often be unreliable.
With reference to the characteristic curve shown in FIG. 1 it can be seen that relatively steep rises of temperature occur at the points on the curve around the start of charge A and around the approach of completion of charge B. The rise in battery temperature at the start of charge can be misinterpreted as the approach of end of charge such that the charging cycle is terminated prematurely, resulting in an undercharged battery.
The temperature of a battery at the start of charging may differ from the ambient temperature of the local environment. An increase in battery temperature, not due to charging, will occur for example if a battery is brought into a warm room from a cold outdoors.
The temperature of the thermal mass of the battery unit will rise until the battery temperature is equal to the ambient temperature of the new environment. The rate of change of battery temperature is proportional to the difference between the battery temperature and ambient temperature. As the battery warms toward ambient there will therefore be a reduced rate of increase in battery temperature due to this thermal mass equalisation process.
If charging is started while the battery temperature is significantly different from ambient temperature then the additional rise in temperature due to the thermal mass equalisation could tend further to confuse the start of charge and completion of charge indications.
To avoid such misinterpretation it is known that a timer may be used to provide a start up period and maintain the charging rate through the early charging stage. As can be seen from the characteristic curve shown in FIG. 1, once the battery is charged past the relatively steep rises of temperature occurring at the points on the curve around the start of charge at A, the completion of charge around B may be determined more easily.
Problems with timers are experienced, however, when an almost fully charged battery is placed in circuit. The initial charging rate continuing for the full duration of the timer start up period would mask the detection of full charge. The battery would therefore overcharge and become permanently damaged.